The present invention relates to generating print-dot data for high resolution printing devices.
Image data may consist of a plurality of multi-bit words, in which each word identifies the brightness of a respective picture element (pixel) of the image. In a full color system, each word may consist of twenty four bits, with eight bits allocated to each of the three primary colors of red, green and blue (RGB). When an image is being displayed, the associated words are loaded into a memory device (referred to as a framestore) which is then continually read in raster order (at 25 or 30 frames per second) to produce a video signal.
Systems are known for converting intensity related pixel data into a form suitable for printing permanent hard copies, in which light absorbent ink is transferred to a suitable image carrying medium, such as chemically treated paper. Unlike phosphor stimulated by electrons, each ink has only one level of light absorbency, therefore, different techniques must be employed to create a full gamut of colors.
A technique, for producing "half tones" in a printing process, is disclosed by Peter Stucki in his book "Advances in Digital Image Processing; theory, application and implementation", published as part of the IBM research symposia series. In particular, a process of "digital screening" is described, from page 205, in which the total image area is divided into a regular pattern of unit areas, each unit area is divided into a pattern of print-dot positions and the darkness of each unit area is determined by the number of print-dot positions which have a dot of ink applied thereto. This approach may be used to convert intensity related pixel data to print-dot data, in which each pixel of the intensity related image becomes an identifiable unit area of the printed image, onto which a plurality of print dots (sometimes referred to as imprints or pels) may be applied.
The print dots could be distributed evenly over each unit area but, particularly with very small dots, the relationship between the amount of ink applied and the area covered is very non-linear. The solution, identified in Mr. Stucki's book, is to cluster a predetermined number of print dots together, so that the image consists of an array of evenly spaced larger dots of variable size, as shown in FIG. 23 and 27 of the chapter "Image Processing for Document Reproduction". However, a disadvantage of this approach is that it reduces the perceived spatial resolution for a given print-dot resolution, therefore, a higher print-dot resolution is required for a given perceived resolution. In practice, when converting from intensity related pixel data to spatially related print-dot data, the resolution of dot clusters should be of similar order to the spatial resolution of the pixels and the number of print-dots in each cluster should be of similar order to the number of intensity levels representing each pixel. Thus, for magazine quality prints, the screening resolution is typically above 130 clusters per inch and the resolution of the print-dots is twelve times that of the dot clusters in both the x and y dimensions.
In a typical full colour printing process, black ink and inks for the subtractive colors of cyan, yellow and magenta are used. A photographic separation is created for each colour, consisting of mutually offset patterns of dot clusters and the shape and position of each cluster is determined by the position of high resolution print-dots. In a system receiving input image data in the form of intensity related pixels, a print-dot generator is provided to calculate the position of each high resolution print-dot for each colour separation.
Dot clusters in the output data and pixels of the input data may have a one-to-one relationship, i.e. the spatial resolution of the dot clusters (formed from a plurality of high resolution print-dots) may be exactly equal to the spatial resolution of the intensity related pixels. Alternatively, each cluster may receive a contribution from many pixels or enlarged images may be produced by generating a plurality of clusters for each pixel.
An electronic graphics system is disclosed in U.S. Pat. No. 4 514 818 (equivalent to British Pat. No. 2 089 625B) assigned to the present Assignee (Applicant) for generating intensity related pixel data and a system embodying the principles disclosed in this patent (and arranged to generate data at a resolution suitable for printing high quality images) is manufactured by the present Assignee (Applicant) and sold under the trade mark "GRAPHIC PAINTBOX". The image, stored in a framestore, is viewed on an RGB monitor and movement of a graphic implement, such as a brush, chalk or air-brush, is simulated by moving a stylus over a touch tablet. The stylus is constructed so that, when manipulated, it generates a signal representing the pressure with which it is applied to the touch tablet and the touch tablet produces a signal representing the x and y co-ordinates of successive points on a stroke made by moving the stylus over the touch tablet. A processing unit receives the pressure and position signals,, along with an indication of the selected implement and the selected color, from which it modifies the values stored in the framestore.
Once an image has been generated, it may be stored on a suitable data carrying medium (magnetic tape or disc etc.) or supplied to a printing device via a suitable data converter. The data converter first of all converts the additive RGB signals, read from the framestore, to subtractive cyan, yellow, magenta and black (CYMB) signals which are then supplied to a print dot generator.
As described in Mr. Stucki's book (page 214) the angle between each array of dot clusters, making up a respective separation, is made as large as possible to prevent Moire patterns, thus, typical displacement angles (with respect to a notional east-west line) are cyan 75 degrees, yellow 90 degrees, magenta 15 degrees and black 45 degrees.
In the aforesaid system, manufactured by the present Assignee/Applicant, image data may be generated for printing magazine-quality images 300mm square, having six dot-clusters per mm, each constructed from high resolution print-dots at seventy two per mm. Each cluster receives a contribution from four adjacent pixels of the intensity related image and a resolution of six dot clusters per mm is satisfactory for producing full colour images for, say, photographs and headings. However, a problem with this known system is that it cannot resolve body text, and line drawings etc. where characters may be less than two millimetres square and line thicknesses substantially less than one mm. Thus, the electronic processing capabilities of the graphics device described above can only be used for full colour images and headlines, each printed in lower resolution dot clusters. Body text and fine line drawings (logos etc.) must be composited on a conventional printing machine, producing a transparency of the artwork, and then combined with the colour separations, as part of a photographic process in which the colour images are said to be "dropped in" around the areas of text. In situations where the text is placed over an image, as is common in advertising brochures etc., it may be necessary to form a matte to block out areas of colour in the separations where text will be added.
It is an object of the present invention to provide an improved system for generating print-dot data. In particular, it is an object of the present invention to provide an electronic system for combining high resolution characters (alpha-numeric text, logos and line work, etc.) with full colour graphics and artistic images.